1. Field of the Invention.
The present invention relates to a densitometer and, more particularly, to a densitometer for use in a photodosimeter system having a high degree of accuracy over a wide range of measurements.
2. Description of the Prior Art.
In my copending application Ser. No. 491,875, filed concurrently herewith, for Photodosimeter Film Badge, there is disclosed a photodosimeter film badge sensitive to non-ionizing radiation which is useful during phototherapy for the treatment of hyperbilirubinemia in the newborn. Such film badge is capable of permitting the measurement of the total irradiance effective in decomposing bilirubin. The film badge undergoes an optical density change as a result or irradiation by the phototherapy lamps and such density change is directly proportional to the time interval of the irradiance. The optical density change occurs very slowly so that the film badge is responsive to phototherapy irradiance over periods extending from a few hours to as many as 96 hours.
Since the optical density of the film changes irreversibly as a function of irradiance, such optical density may be measured directly without any chemical processing of the film. This permits exposure to be monitored continuously. However, in order to derive the full benefits of the film badge of my copending application, it is necessary to be able to measure density extremely accurately, over a wide range of densities.
More specifically, phototherapy for the treatment of hyperbilirubinemia in a newborn typically continues for a minimum of a few hours and a maximum of four days (96 hours). With an irradiance level of approximately 1 mW/cm.sup.2, there is a total exposure, over a period of 100 hours, of approximately 360 joules. Any system used for measuring exposure must measure to an accuracy of about 15 minutes in an exposure interval of 100 hours, requiring an accuracy of about 0.25 percent, or about 1 part in 400. Since the film badge of my copending application exhibits a total density change of about 3.000 in a 100 hour interval, such an accuracy implies that a density change of 0.008 must be accurately measured. Thus, any densitometer must be extremely accurate, the output reproducible, and it must operate over a wide range of densities. However, while many commercial densitometers are available at prices ranging from several hundred to several thousand dollars, no reasonably priced densitometer meets these requirements.